• Snacks - stock up at the grocery store, you'll spend way more across the states in gas stations
• Advil/pain reliever - you'll get headaches staring at the road for that long, and muscles will cramp up
• Water. So so so much water. Soda is fine but bring a few gallons of water
• Get a good charger system for your phone, and make sure you can hook it up to your car reliably

Oh and most important, bring peace of mind by getting your car checked before going. Go in and ask for a full inspection, tires, brakes, fluids, belts. I had a belt burst 8 hours from anywhere and had to sleep in my car, just get that stuff checked now